The present invention relates to a spectroscopic element used in optical machines that use multiple wavelengths. More specifically, the present invention relates to an optical diffraction element that uses a transmission grating.
Optical diffraction elements, of which diffraction gratings are a representative example, are widely used for optical spectrum analysis in spectrum analyzers. Spectrum analyzers need to have a high energy usage efficiency over a wide wavelength range. Reflective diffraction gratings are suited for providing high diffraction efficiency over a wide band range. Also, reflective diffraction gratings are widely used in spectrum analyzers since they provide superior properties with regard to the change in diffraction angle relative to wavelength, i.e., wavelength angle dispersion properties.
However, with reflective diffraction gratings in which the grating interval of the diffraction grating is approximately the wavelength, there can be significant changes in wavelength loss relative to the polarization state of the light. In these diffraction gratings, the wavelength loss properties are complex because they operate in the resonance region, preventing them from being stable across a wide wavelength range (e.g., see Non-Patent Document 1).
With transmission gratings, on the other hand, low polarization-dependent loss (PDL) and high diffraction efficiency can be achieved in a predetermined wavelength range. In transmission gratings having a periodic pattern of grooves, the diffraction efficiency is significantly influenced by the shape of the grooves and the material used to form the grooves. FIG. 1 shows an example of a laminar transmission grating 100 in which the grooves 112 have rectangular cross-section shapes perpendicular to the longitudinal direction of the grooves. It is known that a high diffraction efficiency can be obtained by making the groove depth h adequately deep relative to the groove pitch p. The ratio of the depth of the groove to the pitch of the grooves is referred to as the “aspect ratio” and it can be said that a diffraction grating with grooves having a high aspect ratio (h/p) has a high diffraction efficiency.
However, producing precise grooves with high aspect ratios is difficult in terms of processing technology, and is especially difficult when the groove pitch is small. When the groove shape and the optical properties of the diffraction grating are normalized by wavelength, it can be found that it would be preferable, in order to avoided processing difficulties, to use materials with high indices of refraction to produce transmission gratings with small aspect ratios. In other words, to obtain the equivalent optical properties, it is easier in terms of the production process to form shallow grooves with material with as high an index of refraction as possible rather than to reduce deep grooves with a material having a low index of refraction.
FIG. 2 shows an example of the diffraction efficiency and PDL as they relate to wavelength in a typical laminar diffraction grating. The substrate in FIG. 1 is formed from silica glass and the ridges are formed from Ta2O5, which is a material with a high index of refraction. The groove depth h is 1350 nm, the groove pitch p is 1111 nm, and the groove width is 555 nm. The figure shows that diffraction efficiency is roughly 70% in both TE mode and TM mode over a wide wavelength range of 1350-1750 nm, and the PDL is low, at −0.7-+0.5 dB.
In diffraction gratings, it is well known that when light having a wide wavelength range enters the diffraction grating, higher-order diffraction angles of the incident light having short wavelengths are superimposed with lower-order diffraction angle of the incident light having long wavelengths, preventing wavelengths from being separated. The wavelength range in which a diffraction grating can be used under conditions that do not result in the superimposition of diffracted light based on this phenomenon is referred to as the free spectral range of a diffraction grating. This range is determined by the following condition, where the shortest wavelength is λ1, the longest wavelength is λ2, and the order of diffraction is m.λ2−λ1<=λ1/m (where λ1<λ2)
A diffraction grating can be used under conditions that do not generate the superimposition of diffracted light within the wavelength range defined by this condition.
[Non-Patent Document 1] Tadao Tsuruta, “Ouyou Kougaku 1”, Baifukan, 1990.
Compared to reflective diffraction gratings, transmission gratings have lower PDL characteristics over a wide range, but as the example above shows, the absolute value of PDL tends to increase at a wavelength range with higher diffraction efficiency (1500-1600 nm). Thus, the wavelength range within which the diffraction efficiency is high and PDL is adequately low is not very wide.
Also, when using a diffraction grating in the infrared wavelength range, visible light may be used as a guide light. For example, a light with a wavelength λ1=633 nm and λ2=1550 nm, the condition described above would not be met even when m=1, thus making the light fall outside of the free spectral range. Thus, when these wavelengths are sent to the diffraction grating at the same time and a measurement is performed, there is noise resulting from the superimposition of the wavelengths. This problem may be avoided by using an optical filter to block incident light having unnecessary wavelengths or by using a different optical detector, but this results in a more complex optical system and more complex measuring procedure.